1. Field of the Invention
The invention relates to a device for deriving a clock signal from an electric signal, the electric signal being made up of consecutive signal blocks, a signal block comprising a first block section containing a first signal and a second block section containing a second signal, the first block section of every n-th signal block containing a unique code-word, the device comprising an input terminal for receiving the electric signal, which input terminal is coupled to a first terminal of switching means, which have a second terminal coupled to an input of a phase-locked loop and to an input of a code-word detector which is constructed to detect the unique code-word, the output of the code-word detector being coupled to a control-signal generator, which has an output coupled to a control-signal input of the switching means, the control-signal generator being constructed to generate a control-signal during the time that first block sections are applied to the first terminal of the switching means, and the switching means being constructed to couple the first terminal to the second terminal under the influence of the control-signal. Such a device is employed inter alia in video recorders in order to derive the clock signal from a video signal, for example a MAC signal, read from a magnetic record carrier. However, the device may also be employed in the recording mode of a video recorder when the clock signal is derived from the incoming signal.
The MAC (multiplexed analog components) signals are signals comprising a sequence of lines (signal blocks) with a first block section containing a line-synchronizing signal and a second block section containing the chrominance and luminance information of the video signal. The first block sections further contain additional signals, such as for example the audio signal and other data. The first block sections in the MAC signals generally contain digitally encoded signals. The chrominance and luminance information in the second block sections generally takes the form of an analog signal. One line (signal block) in the MAC signal has a length of 64 .mu.s and the first and the second signal blocks have respective lengths of slightly over 10 .mu.s and slightly less than 54 .mu.s.
The first block section of every n-th signal block, i.e. every 625-th signal block for the MAC signal, contains a unique code-word. This may mean that in addition to the signal already present in the first block section, a unique code-word is included in this block section. Another possibility is that the signal present in this block section takes the form of the unique code-word.
The relevant signal block which includes the unique code-word corresponds to the 625-th and last video line in the MAC video signal which represents exactly one video picture.
The advantages of the MAC signals are: the television picture has a higher resolution, the sound is of a higher quality, and there is (hardly) any crosstalk between chrominance and luminance signal. This last-mentioned advantage is obtained because the chrominance information and the luminance information are stored separately in a second block section.
2. Description of Related Art
The phase-locked loop (PLL) in the known device derives the clock signal from the signal content of the first block sections, which for this purpose are applied to the input of the PLL. To this end the switching means are closed at the instants at which the first block sections are applied to the first terminal, i.e. the terminals 1 and 4 are interconnected, and at other instants the switching means are open. Here the clock signal is derived by means of a "gated PLL". Such a method of deriving the clock signal is known.